A technology known as being related to internal combustion engine torque control is disclosed, for instance, in JP-A-2003-301766. This technology calculates an indicated torque demanded by a driver in accordance with an accelerator opening and determines a target air-fuel ratio in a control device. Next, this technology corrects the demanded indicated torque in accordance with torque efficiency relative to ignition timing and torque efficiency relative to the target air-fuel ratio, and determines a target throttle opening in accordance with a target air amount that is derived from the corrected torque. Further, this technology calculates an intake retard correction amount from the target air amount and engine speed, calculates an ignition timing retard amount from the corrected torque and an estimated torque derived from the intake retard correction amount, and determines a final ignition timing from the ignition timing retard amount and a basic ignition timing derived from an in-cylinder air amount. Moreover, this technology determines a target fuel injection amount from the in-cylinder air amount and target air-fuel ratio. In other words, the technology disclosed in JP-A-2003-301766 sets a throttle opening, an ignition timing, and a fuel injection amount, which are three typical operation amounts, in such a manner as to achieve both the demanded indicated torque, which is demanded by the driver, and the target air-fuel ratio, which is demanded internally by the control device.
Two different modes of air-fuel mixture combustion in an internal combustion engine are known: homogeneous combustion and stratified charge combustion. Homogeneous combustion takes place while air and fuel are uniformly mixed within a cylinder. On the other hand, stratified charge combustion takes place while a rich air-fuel mixture layer is formed near an ignition plug with a lean air-fuel mixture layer formed around the rich air-fuel mixture layer. In homogeneous combustion, the concentration of an air-fuel mixture is uniform within a cylinder so that the air-fuel ratio in the vicinity of the ignition plug is equal to the overall air-fuel ratio (average air-fuel ratio) in the cylinder. In stratified charge combustion, on the other hand, the fuel gathers around the ignition plug so that the air-fuel ratio of a combustion gas near the ignition plug differs from the overall air-fuel ratio in the cylinder.
It is conceivable that the technology disclosed in JP-A-2003-301766 is designed for an internal combustion engine in which homogeneous combustion occurs. As for an internal combustion engine in which homogeneous combustion occurs, the technology disclosed, for instance, in JP-A-2003-301766 can be used to exercise torque control by adjusting three actuator operation amounts, namely, the throttle opening, ignition timing, and fuel injection amount. Torque can be controlled by adjusting an intake air amount, ignition timing, and combustion air-fuel ratio. In the case of homogeneous combustion, however, the air-fuel ratio in the vicinity of an ignition plug coincides with the overall air-fuel ratio in a cylinder. Therefore, torque control can be exercised by determining the intake air amount from the throttle opening and determining the overall air-fuel ratio in a cylinder from the intake air amount and fuel injection amount.
As for an internal combustion engine in which stratified charge combustion occurs so that the air-fuel ratio in the vicinity of the ignition plug differs from the overall air-fuel ratio in the cylinder, proper torque control cannot be exercised by adjusting the above-mentioned three operation amounts alone. If the air-fuel ratio in the vicinity of the ignition plug varies in a situation where the overall air-fuel ratio in the cylinder remains unchanged, the combustion rate varies. As a result, the relationship between torque and ignition timing (hereinafter referred to as the torque-ignition timing characteristic) varies. Therefore, when torque control is to be exercised in this type of internal combustion engine, it is necessary to adjust the air-fuel ratio in the vicinity of the ignition plug (i.e., actual combustion air-fuel ratio) in addition to the overall air-fuel ratio in the cylinder. However, such an adjustment cannot be made by controlling the above-mentioned three operation amounts alone. To achieve a target torque while achieving a target air-fuel ratio, it is necessary to use another operation amount in addition to the above-mentioned three operation amounts.
As regards the technology disclosed in JPA-2003-301766, it can be considered that the demanded indicated torque is a demand concerning drivability, and that the target air ratio is a demand concerning exhaust gas. Drivability and exhaust gas control both are capabilities of the internal combustion engine. In addition, the internal combustion engine has various other capabilities such as fuel economy and knock control. There is a demand for each capability. If, for instance, a target capability is fuel economy, there may be a demand for combustion efficiency enhancement or a demand for pump loss reduction. If the target capability is exhaust gas control, on the other hand, there may be a demand for increasing an exhaust gas temperature or a demand for facilitating a reaction in a catalyst. As described above, various capabilities concerning the internal combustion engine exist, and various demands on different levels may be issued in relation to each capability. However, the technology disclosed in JP-A-2003-301766 meets only some of such demands and still needs further improvement to accommodate a variety of demands about the internal combustion engine.